1. Field of the Invention
This invention relates to a process for the selective catalytic dehydration of n-propanol or iso-propanol (collectively referred to as propanol) to propylene using an improved technology of reactor design and configuration, wherein the reactor train is comprised of a multi-stage single reactor vessel or multiple reactor vessels wherein each stage and/or vessel has different length, internal diameter, and volume than the other stages and/or vessels and in addition the stages and/or reactor vessels are connected in series or in parallel arrangement. Furthermore, this invention discloses an improved means of introducing the propanol feedstock and a heat carrying inert gas to the improved reactor train.
2. Related Information
Propylene is the second most important raw material in the petrochemicals industry after ethylene. It is the primary feedstock in such diverse materials as polypropylene, propylene oxide, propylene glycol, acrylonitrile, epychlorohydrin, etc. This petrochemical feedstock is primarily produced from petroleum resources by the steam cracking of petroleum-derived feedstocks such as heavy naphtha, ethane/propane, or gas condensates. The economics of these processes are greatly influenced by the supply, availability, and price of crude oil and natural gas. In addition, the cracking processes produce a large number of other valuable products and co-products such as ethylene, butylenes, and other hydrocarbons. which have to be recovered and may not be ignored and disposed of as waste. The economics of the propylene production by the steam cracking process thus requires that these co-products be separated and recovered at very high purity suitable for downstream chemicals and polymer applications. This would require very complex processing scheme, high capital investment, and large energy consumption to separate, purify, and provide storage for all the products so that the process can be economically justified. In addition, the success of the petroleum-derived propylene requires that all the by-products be marketed to their respective end users. If a user of propylene were interested in only producing propylene and no other products, the cracking route is not a viable and profitable option. Furthermore, the conventional steam cracking produces large quantities of CO2 (carbon) which is a main component of the greenhouse gas emission.
The dehydration of propanol is a simple and attractive potential route to propylene. Presently, there is no known commercial process for the catalytic dehydration of propanol. Recently, as the biofuels have attracted more attention globally, as prices of crude oil have increased and have become more unpredictable, and as petroleum supply sources have become more unstable and problematic, the propanol dehydration process is gaining interest as an alternative source for the production of chemical- or polymer-grade propylene. In addition, with the threat to the environment and limited resources in some parts of the world, the propanol dehydration process is being increasingly competitive with the traditional steam cracking process. Furthermore, the sources of raw materials for propanol supply are expanding with a resultant decrease in the cost of propanol manufacture thus making it an attractive option for propylene production.
The propanol dehydration reaction basically is characterized by the removal of a water molecule from propanol and as such is highly endothermic. A significant amount of heat (energy) is thus required to initiate and sustain the reactions to completion. Therefore, the choice of the reactor, its design, and configuration are critical aspects of managing the thermal events within the reactor and controlling the operating temperatures within the catalyst bed for an economical process.
Additionally, the economic production of propylene by this process largely depends on the high conversion of propanol feedstock to avoid recovery and recycle of any unreacted propanol. It also requires high selectivity and yield of the propylene product in order to avoid expensive separation and purification of the final product which is needed for chemicals and polymer applications. Furthermore, it is critical to limit the formation of by-products which will complicate the recovery and purification of the product and its downstream applications into high value added chemicals and polymers.
Unlike the ethanol dehydration process to ethylene which has been the subject of many patents and developments and which has been commercially practiced for many years, there have been very few patents and/or technical articles on the dehydration process of propanol to propylene. Recently, however, there has been some activity with several US patent applications having been filed. US patent application 2009/0259086 discloses an integrated process for the production of mono-olefin(s) from a feedstock comprising of at least one aliphatic paraffinic hydrocarbon. In particular, the disclosure claims a process wherein an alcohol stream containing a mixture of ethanol and propanol is dehydrated within a single reactor to the respective same carbon olefins and other by-products. The olefins thus produced are separated from the other by-products and a mixture of ethylene and propylene is produced as the primary product of the process. The disclosure only provides a general layout of a process for co-dehydrating a mixture of ethanol and propanol with no reference to the choice of the preferred reactor for the optimal temperature control required for the dehydration reactions. Neither does the reference disclose the preferred operating reactions required for an economical process which requires high conversion of the alcohols and high selectivity of the olefins products. The reference claims an operating conversion of ethanol and propanol of preferably between only 20 to 60% per pass, which would require a complex process scheme to recover the valuable alcohols and recycle to the dehydration reaction. Finally, there is not provided in this application any experimental results or evidence to support the claims.
US Patent Application 2009/0270668 discloses an invention for a process to co-produce ethylene and propylene from a mixture of ethanol and propanol feedstock. According to this application, the said feedstock is first processed in a purification column to separate the two alcohols. Following this separation, a parallel path is provided wherein each alcohol is separately dehydrated to its respective olefin. The olefins thus produced are then processed to separate them from the by-products and purified to produce the final ethylene and propylene products. The claimed process is very similar to the above US Patent Application 2009/0259086 with the exception that two dehydration paths are provided in this invention instead of a single integrated dehydration reactor in 2009/0259086 disclosure. Similar to the 2009/0259086, the present invention also does not provide any reference as to the choice of reactors preferred for conducting the endothermic dehydration reactions and the management of the temperatures within the individual reactor vessels. Neither does it provide the preferred operation conditions within the reactor vessels.
US Patent Application 2009/0281362, discloses a process where, unlike the above applications, propanol is the only alcohol feedstock and propylene is the only olefin product. According to this invention, pure propanol is dehydrated in the reactor vessel to propylene and other by-products. The product propylene is then separated from by-products and purified to the final product. There is provided no reference to the preferred choice of the reactor. This invention also does not specify the choice of the reactor and its configuration for optimum performance of the overall process.
In view of the above disclosures, the successful development of technology for propanol dehydration to propylene requires that a reactor design be developed consistent with the thermodynamics and kinetics of the dehydration reactions. Foremost, the reactions in this process are highly endothermic which require input of considerable amount of energy to derive the process. Therefore, supply of heat, the management of the thermal processes, and the reactor temperature control constitute important considerations for optimum performance. One aspect of the present invention is a reactor disclosure to address these issues.
With regard to alcohol dehydration reactors which have been proposed and developed in the past, several patents stand out. U.S. Pat. No. 4,134,926 discloses a fluidized bed reactor concept for the dehydration of ethanol to ethylene wherein a portion of the dehydration catalyst is continuously withdrawn from the reactor chamber and regenerated with air in a second fluid-bed regenerator. The hot regenerated catalyst is then mixed with fresh make-up catalyst and recycled back to the primary reactor to provide the endodermic heat of reaction. This reactor concept has not found commercial application due to the complexity of the process, the handling and recycle of large quantities of solid catalyst, and continuous replacement of the lost catalyst because of attrition.
U.S. Pat. No. 4,232,179 describes a reactor train invention in which multiple, adiabatic reactor vessels are connected in series and/or parallel arrangement for dehydration of ethanol to ethylene. This patent further teaches the use of a sensible heat carrying fluid such as steam mixed with the alcohol feedstock prior to feeding to individual reactors. Each reactor is packed with a solid catalyst. The energy required for the reactions is supplied by a fired heater wherein both alcohol feedstock and steam are heated to very high temperatures needed for the reactions to proceed to completion in each reactor stage. This feature, being similar to British patent 516,360, can also result in lower selectivity and yield of the primary product and the formation of problematic by-products. In addition, no distinction is made in this disclosure as to the relative sizes of each reactor and the catalyst bed within that reactor with respect to other reactors and/or catalyst beds which make up the reactor train.
U.S. Pat. No. 4,396,789 teaches an invention which is basically similar to U.S. Pat. No. 4,232,179 with the exception that the reactor train is designed to operate at a design pressure of between 20 and 40 atmospheres. The patent claims that such high pressure operation will simplify the purification of the crude olefin product during the subsequent cryogenic distillation to produce high quality olefin for downstream chemicals/polymer applications.
In all the above processes, the dehydration catalyst is subjected to carbonization and rapid fouling as a result of direct exposure of the alcohol to the high coil surface temperatures in the fired heater which serves as feed pre-heater. This practice would therefore require frequent regeneration of the catalyst bed thus requiring downtime, loss of production, and shortened catalyst life.
The specific goal of the present invention is to provide a novel, adiabatic reactor configuration and process to achieve the desired goals of the invention. Other objects and benefits of the present invention will become apparent from the following disclosure. Furthermore, it is an object of the present invention to utilize available streams already found within a production facility or derived from the operation of the process carried out in the reactor. In this regard, each of the stages within the reactor vessel is independently sized and the quantity of catalyst therein determined to take advantage of a stream from some other reactor or source within the facility or from other stages within the reactor to obtain the highest yield and selectivity from these disparate sources. It is a particular object of the present invention to design each stage of the reactor considering various sources which can used in the reaction at hand.